The big picture: circulation and gas exchange
- Direct exchange between every cell and the environment is not possible in all animals
- special structures for exchange & transport needed
- Internal transport and gas exchange functionally related

Molecule trading
- Small molecules can move between cells and their surroundings by diffusion
- Diffusion is only efficient over small distances
- Molecular trading in animals occurs for every cell!
- gain O2 and nutrients
- release CO2 and waste products
- Simple body plans:
- Complex body plans:

Animals with gastrovascular cavities
- Many animals have lots of cells in contact with environment
- hydras, jellyfish, flatworms
- Central gastrovascular cavity functions to distribute substances
- opening at one end connects cavity to water
- allows fluid to bath inner and outer tissues
- diffusion distance kept short
- Gastrovascular cavity can be highly branched
- extends into tentacles of cnidarians
- covers high surface area of flatworms

Animals with circulatory systems
- Circulatory systems have 3 components
- Fluid transport, via vessels, connects cytoplasm of cells to organs for gas exchange
- Mammals → O2 diffuses over 2 cell layers in lungs to reach blood
- molecule trading works like a circuit

Open and closed circulatory systems (hemolymph vs blood)

Vertebrate Circulatory systems
- Cardiovascular system: blood → heart → vessels
- length of vessels can be staggering
- Arteries:
- Veins:
- Capillaries:
- Hearts of vertebrates have 2+ muscular chambers
- atria are chambers that receive blood
- ventricles pump blood out
- Number of chambers related to form and function

Single vs Double circulation

Evolutionary variation: 3 chambered heart

- Amphibians and reptile hearts have 1 ventricle
- still use double circulation
- Do not always fill their lungs
- pass long periods without gas exchange
- or use another tissue (skin)
- Example: frogs use skin when underwater
- Example: crocodiles shunt blood from lungs when underwater
Mammalian Circulation (2 circuits)
- The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle
- Right ventricle pumps blood to the lungs via arteries
- Blood flows through capillaries into lungs and loads O2 and unloads CO2
- O2 rich blood returns from lungs via veins to the left atrium

Mammalian Circulation (2 circuits)
- Blood flows to left ventricle and pumped to tissues via arteries into capillaries
- First branches supply the heart muscle
- Further branches lead to organs and hind limbs
- O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood
- Capillaries rejoin conveying blood to veins
- O2 poor blood from the head, neck, and forelimbs is emptied into right atrium

Blood flow
- Physical laws govern movement of fluids through pipes
- affect blood flow and pressure
- Blood flows from areas of higher pressure to low
- narrow diameters capillaries decrease pressure
- Blood flow is slowest in the capillaries
- over large cross-sectional area
- necessary for gas exchange

Gas exchange over respiratory surfaces
- Gas exchange = uptake of O2 and release of CO2
- O2 is abundant in the air (21%)
- fairly easily to ‘breath’
- Water is more demanding for gas exchange
- dissolved O2 is variable but always less than air
- Efficient surfaces for gas exchange have evolved

Respiratory Surfaces
- Differs across animal phylogeny
- sponges, cnidaria, flatworm → cells directly
- earthworms, amphibians → skin
- Other animals lack enough surface area
- to much volume to supply with O2
- Respiratory organs evolve
- Gills → aquatic animals
- Trachea → insects
- Lungs → reptiles and mammals

Gills

Trachea in insects


Lungs in mammals, reptiles and birds


Coordination of circulation and gas exchange
- Most animals need to exchange large amounts of O2 and CO2
- During inhalation, fresh air fills in lung
- molecule trading occurs via diffusion
- Blood is then pumped through circuit
- O2 carried in pigments (metal bound to protein)

